Serological diagnosis of untreated early syphilis. Importance of the differences in THA, TPHA, and VDRL test titres.

The treponemal haemagglutination (THA) test has been used for 11 years as a routine test for syphilis at a central serology laboratory. Among patients with untreated early syphilis the antibodies detected by the THA test usually appeared later or were of relatively lower titre than those detected by the reagin tests. This finding was confirmed by follow up of patients and by replies to questionnaires and has been incorporated in a computer program. During 1981, using the results obtained on unpaired specimens and minimal other data, 74 of 123 patients who were finally diagnosed as having untreated early syphilis were correctly identified by the computer. Most of those not selected by the computer were patients with early primary syphilis with little or no circulating antibody or with late secondary syphilis with very high titres.